One type of a conventional system for detecting non-coincidence of codes, which may be used in a cordless telephone system, includes a circuit for detecting non-coincidence between corresponding bits of an input coded signal and a predetermined coded signal, a shift register for storing the results of the non-coincidence detection for each bit of the input coded signal, and a counter for counting the number of bits which are determined not to coincide.
In operation, the bits of the input coded signal are compared in the non-coincidence detecting circuit with corresponding bits of the predetermined coded signal, and the detected results, in the form of high and low level signals, are generated in accordance with the non-coincidence and the coincidence between corresponding bits of the input and predetermined coded signals. The high and low level signals are stored in the shift register, and supplied in series to one inputs of an AND gate pulses are supplied to the other input of the AND gate for permitting clock signals to be supplied to the counter so that the number of the high level signals can be counted. When the counted number is less than or equal to a predetermined tolerable number, the input coded signal is determined to be correct, even if the input coded signal includes errors in some bits.
Another type of conventional system for detecting non-coincidence of codes includes a predetermined number of circuits each determining whether coincidence exists between an input coded signal and one of a plurality of predetermined coded signals, wherein the predetermined number of circuits is determined by a bit error tolerable number, and each possible error coded signals which contains a number of errors which is not greater than the error tolerable number for an input coded signal as supplied to the corresponding coincidence detecting circuits.
In operation, the input coded signal is supplied to the coincidence detecting circuits, and compared therein with the each of the acceptable error coded signals. If any one of the coincidence detecting circuits generates a high level signal representing the coincidence between the input coded signal and one of the acceptable error coded signals, the input coded signal is determined to be acceptable.
However, there are disadvantages in the conventional systems for detecting non-coincidence of codes in that:
(1) the detection takes a long time in the former system because the detected results are supplied in series from the shift register to the AND gate. However, an advantage is that the scale of the system is relatively small, and PA1 (2) the size of the latter system becomes large because the bit error tolerable number can require a large number of coincidence detecting circuits to be provided. However, an advantage is that the detection is carried out in a short time. PA1 means for comparing an input coded signal and a predetermined coded signal to generate a non-coincidence signal including correct an error bits; PA1 means for changing the error bits to correct bits in the non-coincidence signal by a predetermined number of bits, thereby generating a error bit decreased signal; and PA1 means for detecting coincidence between the input coded signal and the predetermined coded signal by checking existence of an error bit in the error bit decreased signal.